This invention relates to apparatus for separating or synthesizing two or more light signals having different wavelengths.
In a wavelength multiplexed transmission system, a single core optical fiber line can be equivalently used as a line having two or more cores so that the versatility of the system is large. However, it is essential to synthesize or separate two or more light signals having different wavelengths. To this end, it is necessary to use a wave separater and a wave synthesizer.
The apparatus utilized in such system for separating or synthesizing optical signals generally employs a dielectric multilayer thin film filter (DMTF), but the DMTF has a polarized wave dependency caused by an incident angle. For this reason, it has been the practice to design the optical system such that its incident angle would be less than 22.5.degree. in which the polarized wave dependency is small. For example, a DMTF 2 and a total reflection mirror 3 are bonded to opposing surfaces of a parallelogrammatic prism 1, as shown in FIG. 1, so as to parallely derive out light signals respectively having wavelengths of .lambda.1 and .lambda.2 while maintaining the incident angle to be less than 22.5.degree.. Various other types of apparatus have been used, but in each of the prior art apparatus it is essential to use a prism or a mirror for the purpose of limiting the incident angle to be less than 22.5.degree.. Moreover, as it is necessary to use special angles, the construction of the apparatus is complicated and expensive.